This prospective case-control study was conducted in six academic centers from 1995 to 1999. Two groups of infants were recruited; infants who developed early-onset GBS disease ("cases") and infants who did not have GBS disease despite being surface colonized ("controls"). Maternal sera and cord sera were collected for serotyping. Medical records of cases and controls were reviewed. A total of 132 cases, 620 heavily colonized (greater than or equal to 3 sites) and 1018 lightly colonized (greater than or equal to 2 sites) were identified. Titers of IgG anti-antibodies to the infecting GBS serotype will be measured. Cases and controls will be contrasted for the presence of antibody at various titers, both in simple analyses and in multivariate analyses that control for potentially confounding variables. An additional study was conducted to evaluate the effectiveness of risk-based intrapartum antibiotic prophylaxis for prevention of early-onset GBS disease. We compared 109 early-onset GBS cases with 207 matched controls without GBS disease. Both cases and controls were from births to mothers with greater than or equal to one risk factor to GBS disease, as defined by ACOG guidelines. Maternal and infant's medical records were reviewed by chart abstractors who were kept blinded to the case and control status. Antibiotic effectiveness was evaluated by calculating the odds ratio for the comparison of the receipt of intrapartum prophylactic antibiotics in mothers of cases vs. mothers of controls.